Processes and rates of channel change following disturbance by debris flows

The Oregon Coast Range is a dynamic landscape, with steep hillslopes mantled by a thin veneer of soil. In this and other mountainous regions of the world, debris flows play a major role in routing sediment and organic matter from steep headwater streams and delivering it further down in the channel network. Debris flows also play an important role in shaping the topography of mountain landscapes by carving the valleys in which headwater streams form (Stock and Dietrich 2003).

When debris flows occur they typically scour steep headwater streams down to the underlying bedrock (Figure 1). These bedrock channels provide the ideal conditions for calculating the rate of sediment and wood accumulation in headwater streams in the interval between debris flows. Studying channels previously scoured to bedrock also provides an opportunity to understand the temporal succession of channel morphology following disturbance, and to make inferences about processes associated with the input and transport of sediment and wood.

Accumulation rates of sediment and wood were measured in 13 streams that ranged from 4 to 144 years since the previous debris flow. All of the channels were located in a 2.5 km² catchment in an old growth forest (Figure 2), and the timing of these events was determined by the tree ring record (dendrochronology) of trees growing in the valley bottoms of debris flow prone channels. It was possible to use dendrochronology to date the time since the previous debris flow because any trees growing in the valley bottom would have been scoured away by the most recent event. Additional evidence that trees growing in the valley bottoms were a good indication of the time since debris flow was that even-age stands of trees, which were younger than the forest on the surrounding hillslopes, were present in the valley bottoms. This is due to the catastrophic removal of trees during a debris flow and subsequent regrowth of vegetation after the event.

The volume of sediment stored in the channels was quantified by detailed field measurements of the length, width and depth of patches of sediment stored in the channel. The volume of wood was also quantified, and all wood > 20 cm diameter and > 2 m in length that was in contact with the channel was measured. Results from this investigation indicate that the volume of sediment and wood stored in a channel was strongly correlated with the time since the previous debris flow (Figures 3 and 4); thereby decreasing the proportion of the channel length with exposed bedrock through time. Downed wood played an especially important role in the routing and storage of sediment in these steep channels. Wood functioned to increase the storage capacity of the channel, and trapped >70% off all stored sediment. In the absence of wood, these steep channels have the ability to transport the majority of sediment that enters from the surrounding hillslopes.

The observed linkage between wood abundance and the sediment storage capacity of the channel highlights the tight coupling between channel morphology and forest vegetation. This coupling is of critical concern in the Oregon Coast Range, where clearcut logging and fires frequently remove vegetation along headwater streams. In the absence of wood, channels that have been scoured to bedrock by a debris flow may lack the capacity to store sediment and could persist in a bedrock state for an extended period of time. If these channels are not storing sediment it means that they have become a chronic source of sediment to downstream areas. In addition, debris flows have a greater likelihood of occurrence when vegetation has been removed because of a loss of rooting strength in landslide prone areas where debris flows originate. Numerous studies have found that roots of the large trees are vital for anchoring soil on the steep hillslopes (e.g., Montgomery et al. 2000; May 2002).

With an adequate supply of wood, headwater streams have the potential of storing large volumes of sediment in the interval between debris flows. This is important because headwater streams are very abundant and are tightly connected to hillslope sources of sediment. In addition to the routing and storage of sediment, headwater streams also provide important services to downstream ecosystems, such as regulating the timing and magnitude of streamflow, organic matter transport, and water temperature inputs (Gomi et al. 2002). Headwater streams also provide unique and predator-free habitats for numerous amphibians and invertebrates, and represent an important food source to downstream fish communities (Wipfli et al. 2007). Channel morphology largely determines the type of habitat available for organisms, and this study documented the temporal succession of channel change following a debris flow (Figure 5).

Immediately following a debris flow the channel was predominantly bedrock, with almost no sediment or wood in storage. These bedrock channels have the capacity to rapidly transport nutrients and organic matter, and have limited structural complexity to provide habitat for organisms. Fifty years post debris flow, small discrete patches of sediment were stored behind individual logs, but the channel was still predominantly bedrock. One hundred years after a debris flow almost half of the channel length was still exposed bedrock; and by 150 years discrete patches of sediment had coalesced to form larger, more continuous patches. Beyond this point in time (and beyond the maximum age of channels in the study area), the channel would be predicted to have an almost continuous cover of sediment with very little exposed bedrock. Channels in this condition are structurally complex, provide diverse habitats for organisms, and are very retentive of nutrients and organic matter.

Glossary:

Debris flow: a channelized mass flow that rapidly travels down steep a channel and is composed of a thick mixture of sediment, wood and water.

Dendrochronology: is the analysis of the growth rings of trees with the objective of dating events in the relatively recent past.

Headwater stream: The upper extent of the channel network, characterized by interactions among hydrologic, geomorphic, and biological processes that vary from hillslopes to stream channels and from terrestrial to aquatic environments (Hack and Goodlett 1960). Headwaters differ from larger rivers due to their close coupling to hillslope processes, greater temporal and spatial variation, and their need for different means of protection from land use (Gomi et al. 200).

• Gomi, T., Sidle, R.C., and Richardson, J.S. 2002. Understanding processes and downstream linkages of headwater systems. Bioscience 52(10): 905-916.
• Hack, J.T. and Goodlett, J.C. 1960. Geomorphology and forest ecology of a mountain region in the central Appalachians. U.S. Geological Survey, professional paper no. 347.
  
  
  
• May, C.L. and Gresswell, R.E. 2003. Processes and rates of sediment and wood accumulation in headwater streams of the Oregon Coast Range, U.S.A. Earth Surface Processes and Landforms 28(4): 409-424.
• May, C.L. and Gresswell, R.E. 2004. Spatial and temporal patterns of debris flow deposition in the Oregon Coast Range, U.S.A. Geomorphology 57:135-149.
• May, C.L. 2002. Debris flows through different forest age classes in the central Oregon Coast Range. Journal of the American Water Resources Association 38(4): 1097-1113.
• Montgomery, D.R., Schmidt, K.M., Greenberg, H.M., and Dietrich, W.E. 2000. Forest clearing and regional landsliding. Geology 28(4):311-314.
• Stock, J., and Dietrich, W.E. 2003. Valley incision by debris flows: Evidence of a topographic signature. Water Resources Research 39(4), 1089, doi:10.1029/2001 WR001057.
• Wipfli, M.S., Richardson, J.S., and Naiman, R.J. 2007. Ecological linkages between headwaters and downstream ecosystems: Transport of organic matter, invertebrates, and wood down headwater channels. Journal of the American Water Resources Association 43(1): 72-85.